The present invention relates to a hydraulic vehicle accessory drive power steering system.
It has been known from, for example, U.S. Pat. Nos. 3,659,567, 4,179,888 and 4,446,697 that a single hydraulic motor is used to hydraulically drive an automotive power steering mechanism and a hydraulic fan motor. In the prior art hydraulic drive system disclosed, for example, in U.S. Pat. No. 3,659,567, the hydraulic fan motor and the power steering mechanism are connected in series to the single hydraulic pump. The system includes a bypass line extending in bypassing relationship to the fan motor. A bypass valve is disposed in the bypass line and, when the power steering mechanism is operated, is responsive to a pressure difference across the power steering mechanism to cause the hydraulic fluid to flow to the power steering mechanism in preference to the fan motor.
In the prior art hydraulic drive system, however, the bypass valve is opened and closed merely in response to operations of the power steering mechanism. Therefore, when the bypass valve is opened in response to an operation of the power steering mechanism, it is possible that the hydraulic fluid is fed to the power steering mechanism under different pressures with a result that the mechanism produces discontinuous steering power or torque which gives the operator a physical feeling of disorder and discontinuity of the steering power. On the other hand, if the operator frees the steering wheel to abruptly remove his steering force therefrom, the bypass valve is suddenly closed with a resultant abrupt increase in the supply of the hydraulic fluid to the fan motor. Thus, the fan motor is suddenly driven again to produce noise.